Polyester resins, and polyethylene terephthalate copolyesters in particular, may suffer from thermal instability, as evidenced for example by bubble formation when maintained at the relatively high temperatures needed for extrusion, and the like. This bubble formation, and other defects seen during thermal processing, result at least in part from the presence of off-gas in the form of CO2 and CO. This thermal instability is a function both of the thermal processing temperature and the length of time the polymer is kept at an elevated temperature. These bubbles, when present, may be observed by the naked eye, and lead to streaks in molded and molded articles and films. Color and clarity are both important for copolyesters intended for demanding applications such as signage, and a copolyester product ideally exhibits both high clarity and low yellowing.
To control the yellow hue, polyester polymers may be prepared using small amounts of titanium catalysts and low reaction temperatures. However, the resulting polymers may still be yellower than the intended use will allow, as evidenced by their b* values.
Yet another problem with polyethylene terephthalate copolyesters is that unintended diethylene glycol is produced during polymer production, which is then incorporated into the growing polymer chain. This DEG level may vary significantly during a production run, resulting in a copolyester having inconsistent or undesired properties.
U.S. Pat. No. 5,558,912 discloses a photo-stabilized polymer blend comprising: at least one polyethylene terephthalate-based (PET) copolymer comprising 1,4-cyclohexanedimethanol and, as an ultraviolet absorber, at least one compound selected from the group consisting of cyclic imino esters.
U.S. Pat. No. 5,681,918 discloses a three step process for preparing copolyesters exhibiting a neutral hue, high clarity, and increased brightness. The process involves reacting terephthalic acid, ethylene glycol, and 1,4-cyclohexanedimethanol in a feed mole ratio of total glycols to dicarboxylic acid of 1.7:1 to 6.0:1, at a temperature of 240° C. to 280° C., and a pressure of 15 psig to 80 psig, to form an esterification product; adding a polycondensation catalyst selected from titanium, germanium, antimony, and combinations thereof, and a toner; and polycondensing the product at a temperature of 260° C. to 290° C. and a reduced pressure to form a high molecular weight copolyester. The use of titanium as a polycondensation catalyst is exemplified.
U.S. Pat. No. 6,156,867 discloses a polyester polymerization catalyst, comprising a solution containing an aluminum compound and an alkali compound, with water or an organic solvent or a mixture consisting of water and an organic solvent as the medium. Also disclosed is a polyester production method, in which the product obtained by an esterification reaction or ester interchange reaction between an aromatic dicarboxylic acid or any of its ester forming derivative and a diol is polycondensed, to produce a polyester, comprising the use of the polymerization catalyst containing an aluminum compound. The catalyst is said to provide a polyester excellent in processability and to overcome such problems as spinneret contamination, filtration pressure rise, filament breaking, film breaking and foreign matter production in the production process of products such as fibers, films, resins and bottles.
U.S. Pat. Publn. No. 2007/0066735 discloses a polyester composition containing: a) aluminum atoms; and b) alkaline earth atoms or alkali metal atoms or alkali compound residues such as lithium atoms; and c) particles comprising titanium, zirconium, vanadium, niobium, hafnium, tantalum, chromium, tungsten, molybdenum, iron, or nickel atoms or combinations thereof, where the particles improve the reheat rate of the polyester composition. The polyester polymer compositions may also contain phosphorus catalyst deactivators/stabilizers.
U.S. Pat. Publn. No. 2007/0066791 discloses a polyester polymer composition containing polyester polymers such as polymers having repeating ethylene terephthalate units, aluminum atoms in an amount of at least 3 ppm based on the weight of the polymer, the polyester polymers having an It.V. of at least 0.72 dL/g obtained through a melt phase polymerization and a residual acetaldehyde level of 10 ppm or less. Also provided are polyester polymer compositions containing polyester polymers and: (i) aluminum atoms (ii) alkaline earth metal atoms or alkali metal atoms or alkali compound residues, and (iii) a catalyst deactivator such as a phosphorus compound. The phosphorus compound is added to the polyester melt either late in the polycondensation or upon remelting a solid polyester polymer. The polyester polymer exhibits good L* brightness, clarity, and low levels of acetaldehyde generated upon melting.
U.S. Pat. Publn. No. 2008/0027206 discloses a process for feeding metals to one or more melt phase process lines for the production of polyester polymers in which two feed streams of different types or amounts of metals are provided so that the metals can be added to the process at different points, and so that the amount or ratio of metals provided to the process can be varied over time. The document discloses that adding lithium within the esterification zone, or at least before 50% conversion, will reduce the diethylene glycol (DEG) or DEG residue content in the polyester polymer produced by the melt phase process.
It would be desirable to provide a copolyesters made using a highly active catalyst system that has improved thermal stability during processing, is less yellow as evidenced by a lower b* value, and may have reduced diethylene glycol levels, if desired.